Solving the Plymouth Noise Testing Mystery

by Chris Kapsambelis

Recently, I wrote about the curious phenomenon, where the wind turbine noise appeared softer than the ambient background, a seemingly impossible condition.

Since then, I have come across evidence that might explain how this is possible. In order for the Future Generation Wind (FGW) project to gain a Special Permit from the town of Plymouth, MA, they submitted the results of a study by Atlantic Design Engineering (ADE) which included physical background ambient sound level (L90) measurements from the same locations, nearest the proposed turbine sites, as the post construction Tech Environmental study, on which the strange occurrence of wind turbine noise quieter than the background ambient, was recorded.

In each and every case, the preconstruction background ambient (L90) levels are significantly much quieter than the post construction L90 levels. For the date of September 20, 2017 at 1:30 to 1:45 AM, on which the nonsensical results of the post construction study were observed, the difference is almost 18 dBA, and if the 31 dBA preconstruction level is the real value of the background ambient, then the post construction turbine-on measurements make sense.

So, what is the explanation for this large difference in ambient measurements?

Originally, I thought that there must be an unidentified source of noise present during the turbine-off measurements. However, the fact that this is not an isolated case, but it is consistently found for all locations involved, means the unidentified source theory is not credible. In retrospect, one must conclude that the extra noise with the turbines off, must be generated by the turbines themselves, even though they are turned off.

The traditional MassDEP noise monitoring protocol is based on the fact that when an industrial source of noise is turned off at the quietest part of the night (12:00 AM to 4 AM), the background ambient can be measured accurately because it is normally free from noise from all sources. Including the source under investigation. It appears, however, that in the case of wind turbines, at hub-height wind speeds, this is not true.

It is well known that wind turbine noise is made up of two components. The continuous noise attributed to the operation of gears and the generator that resemble other industrial sound, and the pulsating aerodynamic noise that results from the wind striking the 500-foot-tall wind turbine structure. The pulsating nature of the aerodynamic sound component is the result of the rotating rotor which modulates the sound level. Apparently, when the rotor is stopped from rotating, and the modulation stops, the aerodynamic noise does not disappear in its entirety. Depending on where in the rotating cycle the rotor come to a stop, the aerodynamic noise must continue at a steady sound level.

It is well known that when the wind strikes a standing structure, (house, tree, statue, wall, etc..) aerodynamic noise can be heard. Wind turbines are designed to capture the wind at high heights (500 ft,) where the wind is expected to be strong, as much as 8 times stronger than on the ground. While this design results in generating higher levels of energy, it is also conducive to generating higher levels of noise, even when the turbines are not running.

A plausible explanation for the observed difference between the high post construction background ambient sound levels, and those measured before the turbines were installed, is that the aerodynamic sound generated by the high, hub-height, wind speed causes a substantial increase in the background ambient sound level.

Given the observations above, the MassDEP traditional On/Off approved protocol is not appropriate for wind turbine noise assessment, and must be abandoned in favor of an alternative that avoids this gross error. Either the preconstruction measurements for ambient should be used, or measurements from an alternative comparable site away from any influence of wind turbine noise, and approved by MassDEP, should be used.

While current MassDEP policy allows for the use of an alternative site, away from wind turbine influence, to be used, up to now, this procedure has been rejected by MassDEP.

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